In a known method for the prevention of corrosion, a thermoplastic film is laminated on the inner wall and outer wall of a metal can. For example, Japanese Patent Unexamined Publication No. 7-227946 discloses a polyester film to be laminated on a metal material for food cans. This polyester film is superior in scratch resistance, and a metal plate laminated with this film (hereinafter to be referred to as a film-laminated metal plate) is free of degradation of a product value due to the occurrence of scratches on the film surface in, for example, a can manufacture step comprising forming a metal plate into a cylindrical shape and seaming lids on the upper and lower openings of this cylinder, when the film-laminated metal plate is delivered or subjected to seaming.
The above-mentioned polyester film is superior in resistance to seaming and associated with less elution of oligomer from the film during heat treatment such as retorting applied after can manufacture and packing food therein. Consequently, this film serves well as a film to be applied to the inner wall of a metal container.
Cans for soft drinks include a so-called three-piece can wherein lids are set on the upper and lower openings of a cylinder formed using a metal plate, and a so-called two-piece can wherein a metal plate is drawn deep to form a container part and a lid is seamed onto the upper opening of this container part.
In the case of a three-piece can, a film-laminated metal plate is formed into a cylindrical shape, but in the case of a two-piece can, a film-laminated metal plate undergoes a draw-wipe forming.
Consequently, the film to be laminated on a metal plate that is used to make a two-piece can is required to have superior moldability that allows the film to be molded following the shape change of the metal plate. At the same time, the film is required to show superior adhesion to the metal plate. When the film has insufficient moldability or poor adhesion to the metal plate, the film may come off from the metal plate (i.e., delamination) or the film may be broken during manufacture of the container part of a two-piece can.
A film-laminated metal plate is formed into the container part of a two-piece can by repeated ascending and descending of a punch used for drawing. Therefore, the film constituting the inner wall of the container is required to be easily releasable from the punch.
Moreover, the film constituting the outer wall of the container is required to be free of whitening or precipitation of oligomer due to heat treatment for retorting or printing, which is applied after can manufacture including drawing and wiping.